1. Field of the Invention
The present invention relates to four-port directional filters and, more particularly, to a dual mode filter system that utilizes a pair of filters connected in phase quadrature between an input and output waveguide and includes a novel switching device incorporated into one of the filters for directing and switching the propagation of the output signal between the two ports of the output waveguide.
2. Description of the Prior Art
Filters used in the processing of electromagnetic waves are well known in the art. Many employ a common construction which includes a series of cavities constructed from a cylindrical wall that is closed off at both ends. Divider walls are sometimes utilized within the cylindrical wall to form additional cavities, one behind the other. These walls are generally made from an electrically conducting material, such as aluminum, brass or silver-plated steel.
The filter also includes a coupling device for coupling power from both a longitudinal component and a transverse component of the wave being filtered. In the case of a transverse electrical wave, the coupling device couples power from the magnetic field of the wave. When an electrical wave is being filtered, the coupling device usually takes the form of distinctly shaped apertures formed on the divider walls for coupling the electromagnetic power between adjacent cavities. Alternatively, if a transverse magnetic wave is being filtered, then the coupling device couples power from the longitudinal and transverse component of the electrical field of the transverse magnetic wave. The coupling device used with such a wave generally utilizes probes, rather than slots, for coupling the components of the electrical field into the cavities of the filters. The dimensions of the cavities and the configurations and positions of the apertures or probes are selected to provide the desired band pass characteristics to the signals (waves) propagating through the filters.
One problem associated with such filters occurs when several signals are being filtered simultaneously by a number of filters. Signal distortion can occur if the signals outputted by individual filters interfere with the operation of adjacent filters. The solution to this problem requires the various filters to be electrically isolated from each other to avoid improper signal combination.
One solution to this isolation or signal distortion problem includes the development of the four-port filter. These filters allow for connection of several filters to a single manifold without interference between filters. This form of filter includes two input ports and two output ports. The filter is designed so that a signal applied to one of two input ports exits from only one designated output port.
One particularly useful four-port filter system is disclosed in our issued U.S. Pat. No. 4,780,694 for a Directional Filter System, the disclosure of which is hereby incorporated by reference. That particular filter system utilizes a pair of matched filters interconnected between an output and input waveguide. Both waveguides are provided with coupling means for coupling power from both a longitudinal and transverse component of a magnetic field of a transverse electrical wave. That particular filter system creates a phase quadrature relationship and results in the generation of an output wave or signal which propagates through one of two ports of the output waveguide. The filter operates such that a wave inputted into one of the input ports will produce an output wave that will propagate through only one particular output port. The operation is reciprocal in that a wave generated through the other input port will exit only from the other output port.
Our previous filter system can be utilized for numerous applications, including use on satellites and other communication systems. In order to change the propagation of the output wave from port to port of the output waveguide, an appropriate external switch and housing must be included. This external switch can be implemented to select the correct input port of the input waveguide, in order to switch the signal propagated when desired. In some applications, numerous directional filters must be used and each must have its own external waveguide switch and housing. In satellite use, the need for additional switches and housings usually increases the overall weight of the satellite, thus increasing the cost to deploy and maintain the satellite in orbit. Therefore, it would certainly be beneficial and advantageous if the filter system could utilize a switching device which eliminates the need for a separate waveguide switch housing. The resulting filter system would realize at least a nominal weight savings for each filter that is utilized. However, such a switching device would have to perform without any negative impact on the integrity of the filter system.